In the art, multi-level converters are known to be used in order to reduce harmonic distortion in the output of voltage source converters. A multilevel converter is a converter where the output voltage—or, in case of a multiphase converter, the voltages—can assume several discrete levels, as can be seen for example in DE10103031.
In WO 2008/067785, a multi-level converter according to DE10103031 is disclosed which in addition comprises at least one inductor in each phase leg as well as regulating means to regulate a circulating current flowing through the phase legs, i.e., the current that closes between the phase legs but does not enter the AC grid through the AC terminal.
If the circulating-current is controlled, as described in WO 2008/067785, the voltage rating of the power electronic switches of the converter must allow for the extra voltage needed to regulate the circulating currents in the desired manner.
It is also known to add a zero sequence third harmonic to an AC voltage. This means that harmonics at three times the fundamental frequency are added to the converter AC voltage reference. Such an addition lowers the peak voltages of the converter and thereby the efficiency of the converter power is raised. However, these harmonics are normally not allowed in an AC grid and therefore they have to be removed. This removal is typically performed through using a transformer connected between the AC grid and the converter.
However, it is in some cases of interest to remove this transformer. This means that the zero sequence third harmonic has to be removed in some other way.
WO 2004/017505 describes one such alternative solution for removing these harmonics without the use of a transformer. This document describes the use of a phase conductor wound around an iron core and placed between a converter and the AC grid instead of a transformer. This phase conductor then takes care of the third harmonics removal.
It is however of interest to combine regulating of circulating currents with zero sequence third harmonic removal. This will limit the number of elements used for performing these two functions separately.